CS 61C, Midterm #2, Spring 1997 CS 61C. Spring 1997 Midterm #2. Professor Brian Harvey

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1 This exam was worth 24 points, and was open book. Problem #1 (4 points) CS 61C Spring 1997 Midterm #2 Professor Brian Harvey Translate the following procedure to MAL. (Assume that the argument and return value for baz() are integers.) int foo(int x, int y) { return x+baz(y+1); } The arguments and return value are in the standard registers. Don't use unnecessary stack space. Problem #2 (4 points) Mark each of the following true or false. (assume that we are talking about a MIPS system running C or C++.) It will cause a runtime error if a procedure returns a pointer to an array declared as static within the procedure If the compiler chooses to put local variables on the stack, it will need a bigger stack frame that if it chooses to put those local variables in registers $16-$23. The expressions *p and p[0] have the same meaning if p is a pointer to an array of integers The expressions *a and a[0] have the same meaning if a is an array of integers. Problem #3 (2 points) True of false: A branch instruction poses only control hazards to the pipeline, not data hazards. Explain your answer. Problem #4 (2 points) CS 61C, Midterm #2, Spring 1997 Suppose we write computer programs to do each of the following computations in floating point arithmetic. Indicate which of the three is most likely to compute the mathematically correct answer, and which is least likely to compute the mathematically correct answer. most likely least likely (((1 + 1/2) + 1/4) + 1/8)... CS 61C 1

2 most likely least likely most likely least likely 1/0 Problem #5 (2 points) 1/r^2 where r is the distance from an electron to the nucleus of its atom, in meters The following MAL code takes a nonzero value in $8 and computes some function of that value in $9. Describe that function in one English sentence. sub $9, $0, $8 and $9, $9, $8 Problem #6 (2 points) Here is some C/C++ code: int x[10]; void foo(void) { int y[10]; } y[3] = x[3]; (The procedure doesn't do anything very interesting.) In the MAL translation of the body of the procedure there will be a LW instruction to get the value of x[3] followed by a SW instruction to store that value into y[3]. For each of these instructions, we want to know the earliest stage in the compilation process at which the final numeric values of the fields in those instructions are known. For the LW: by the C compiler by the assembler by the linker For the SW: by the C compiler by the assembler by the linker Problem #7 (2 points) In a multiprocessing system, it is best to use interrupts for both the keyboard (receiver) and the screen (transmitter). But given the following two possible alternatives, which would be worse? Explain why, in at most two sentences. A. The system uses interrupts for keyboard input, but polling for screen output. B. The system uses polling for keyboard input, but interrupts for screen output. Problem #8 (4 points) CS 61C, Midterm #2, Spring 1997 Suppose we have a mouse attached to our MIPS computer. It has a control register and a data register, like the other devices we've seen. You are going to use interrupts to handle Problem #4 (2 points) 2

3 the mouse. The control register, at 0xffff0010, looks just like the others: I R unused E The data register, at 0xffff0014, has four input bits: U D L R unused P N T T Whenever the mouse is moved a little bit, it generates a signal in at least one (and at most two) of four directions: up, down, left, right. It also turns on the Ready bit, which is turned off when you read the data register, just as in the case of the receiver. (If the mouse is moved a lot, each little bit of the motion generates a signal.) We keep two global variables, mousex and mousey, that hold the mouse's horizontal and vertical position. When the system starts these variables both contain zero. You are to keep track of the mouse's position by adding or subtracting 1 to/from these numbers, according to which bits are set, on each interrupt. To simplify the coding, assume that the mouse is the only device that will interrupt, so your interrupt handler can assume that the mouse has moved. To simplify the problem further, ignore the Up and Down bits, and ignore the vertical position. Just keep the horizontal position correctly. (Pretend that your partner is writing the vertical part!) You must write the intrp procedure:.kdata mousex:.word 0 mousey:.word 0 intrp:.ktext Problem #9 (2 points) Instead of having AND, OR, and NOT gates to work with, suppose you are given only NAND gates. A NAND gate has two input wires and one output wire; the output has the value not (a and b) where a and b are the inputs. We'll represent a NAND gate with this symbol: Problem #8 (4 points) 3

4 Using only NAND gates, build a circuit that computes the AND function. (Two gates should be enough.) Solutions Problem #1 foo: addi $29, $29, -8 sw $16, 0($29) sw $31, 4($29) Problem #2 1) F 2) T 3) T 4) T Problem #3 add $16, $0, $4 addi $4, $5, 1 jal baz addi $2, $2, $16 lw $16, 0($29) lw $31, 4($29) addi $29, $29, 8 jr $31 False. No explanation was given. Problem #4 1) least likely 2) least likely 3) most likely Problem #5 The function produces the greatest integer of the form 2^N, where n is a nonnegative integer, that evenly divides the number in $8. For example, the function will return 1 for any odd integer, 4 for 12, 2 for 14, etc. Problem #9 (2 points) 4

5 Problem #6 For the LW: by the linker For the SW: by the C compiler Problem #7 B. Screen output is on average much faster than keyboard input, therefore if the CPU is required to wait for one or the other, it might as well wait for the faster device. Problem #8.kdata mousex:.word 0 mousey:.word 0.ktext intrp: plus: minus: exit: lui $26, 0xffff lw $27, 0x14($26) andi $27, $27, 1 bnez $27, plus lw $27, 0x14($26) andi $27, $27, 2 bnez $27, minus b exit lw $27, mousex addi $27, $27, 1 sw $27, mousex b exit lw $27, mousex addi $27, $27, -1 sw $27, mousex b exit mfc0 $26, $14.set noat lw $1, 0($29).set at addi $29, $29, 4 rfe jr $26 Problem #9 Problem #6 5

6 Here is the truth table: a b c d Posted by HKN (Electrical Engineering and Computer Science Honor Society) University of California at Berkeley If you have any questions about these online exams please contactexamfile@hkn.eecs.berkeley.edu. Problem #9 6